Hum/whine/whistle controlled by throttle, plus audible at idle

Hi guys, I’m trying to work out what this humming/whining/whistling noise is.

YouTube Videos:

This is a DCT 2012 car with now 48k miles on it, serviced meticulously, owned by myself since 2019. It has sounded the same and like-new for all these years, until July/August last year, soon or straight after a service.

Since then it has had a few new noises which I’ve been unable to get answers on:

1) A humming sound at idle which sounds a bit like alternator whine. From the underside of the car, you can hear it clearly (listen out for a tea kettle type of noise in the video) and it’s from somewhere in the engine bay or close.

Idler pulley and both tensioner pulleys have been replaced with new Genuine BMW items, plus both belts. No improvement. Engine mounts were replaced with RBs in 2021, 7k miles ago (Genuine BMW mounts).

2) The other noise (which may be related to the one at idle, but may be a different issue) is a hum/whistle while driving, controlled specifically by the throttle pedal. The volume and onset of this noise is controlled by the throttle pedal, especially audible when in 4th/5th/6th at about 30-35 MPH. Its pitch DOES change with revs, but you only hear it if on throttle (5-20% throttle approx.) When you are hearing it, applying more throttle increases its volume, and letting off a bit decreases its volume. Back off the throttle completely and it goes away.

I am aware of the ‘plenum whistle’ issue (gasket where ICV hose connects to plenum) but this noise sounds less whistle-like than what I’ve heard in other people’s videos. Mine is more like an electrical hum/whine noise. It might be that same issue, but I’d be surprised.

3) The engine itself sounds far more whiney and loud when revving (at idle and when driving), since the last service (or around that time). You would assume it’s a bad pulley but I’ve replaced those as mentioned. PS pump seems ok as the noise doesn’t change when moving steering wheel, AC works fine, I’ve had no alternator/electrical issues/warnings. I do feel like all of the mentioned noises start after 5+ mins of driving when the drivetrain has warmed up a bit.

4) One more thing is a drivetrain clunk I hear in 2nd gear only (seemingly), when I am crawling at about 10 MPH and I go on/off throttle, I hear a clunk/crunch sound from somewhere up front, as if an engine mount is broken or something. Not sure if it’s related, but it’s strange how all of these sounds have started at roughly the same time. (Engine mounts replaced 10k miles ago).

If anyone has experienced this before and/or might know what is causing the noise(s), I’d appreciate your input.

Thanks in advance.

YT vid sounds just like the beginnings of Diff whine either caused by ring and pinion gear backlash (due to wear) or the diff nose bearing (change the fluid). The whistle on the video can be heard increasing in volume as the camera passes the passenger side (in Britain) down pipe which indicates to me the header doughnut seal could be leaking slightly or the two down pipe to header nuts need tightening. The clunk is either the drive shaft center bearing coupling or the driveshaft guibo IMO. Search the forums for more information on exactly these topics.

DrFerry wrote

YT vid sounds just like the beginnings of Diff whine either caused by ring and pinion gear backlash (due to wear) or the diff nose bearing (change the fluid). The whistle on the video can be heard increasing in volume as the camera passes the passenger side (in Britain) down pipe which indicates to me the header doughnut seal could be leaking slightly or the two down pipe to header nuts need tightening. The clunk is either the drive shaft center bearing coupling or the driveshaft guibo IMO. Search the forums for more information on exactly these topics.

Thank you for your reply. All very sensible suggestions.

I have done lotttsss of reading and research and am aware of all of the potential issues that you have mentioned, and of course any of them could be relevant here, but I guess I need to get to a specialist who can do a more serious job of trying to diagnose it all than my usual specialist who isn't really taking it seriously.

In the last service in July, the following things were done:

- Engine oil+filter (Castrol 10W60 as usual, Genuine BMW filter)
- Coolant replacement
- Diff fluid (Castrol like I've been using for the last 5 years, not the BMW one with FM)
- Brake fluid replacement
- Microfilters replacement
- New battery from BMW (and registered)

The noises all started either immediately or soon after the service, so not sure if they are related or it's all a coincidence.

I'm mentioning this to refer to your list of possible causes. Diff whine for the first time ever right after new fluid? I hope they didn't underfill it? I did ask them about it but they are adamant it was all done properly (and they've done it on my car before). Plus the noise really sounds like it's coming from the front of the car (although I'm not 100% sure about this).

I am also wondering about the downpipe>manifold connections, either in terms of gaskets or bolt tightness. Bit weird since they were fitted with new gaskets only 3 years (6k miles) ago, but I will definitely get them looked at. Interestingly, the noise is actually louder on the driver's side (UK), the video makes it hard to hear as well as in person.

My other thought is the alternator but it seems to be working perfectly. A bearing in it, maybe?

CB or guibo are definitely possible causes of the clunk, just a bit strange on a 48k mile car that I only drive on weekends and do 2.5k miles a year in, treating with max respect and mechanical sympathy. I'm wondering about trans/diff mounts.

My aim right now is to book in with a new specialist to look at it all in detail, but what I would love to know is if anyone who had confirmed "plenum whistle" can tell me if theirs sounded like what mine does in my videos, before they fixed it and confirmed the noise to be gone. Then I'll target that.

Any further input is greatly appreciated.

Thanks.

IF the mechanic isn't taking you seriously go elsewhere. I suggest going to Evolve Automotive who are highly familiar with the E9X M3 platform for diagnosis and analysis. They're in Luton so not too far away from you. The correct OEM E9X M3 diff fluid contains the required FM limited slip additives. Most other generic fluids don't contain FM so it's advisable to check and verify what type of viscosity and brand was used during the service. So, the onset of the noise is possibly not a coincidence. The E9X M3 alternator does make a characteristic whine and clunk when the pulley bearings are going out. The pulley also contains a centrifugal clutch which I have posted about before on these forums. Besides the alternator, the serpentine belt tensioners, as well as the PS pump, and PS pulley as well as the A/C compressor pulley can all make whining or whirring noises. Listen under the hood to better identify where the whistling or whining maybe coming from. Another trick is to use a long screwdriver with the tip placed on each ancillary and your ear touching on the handle end to use it as a sort of stethoscope to locate the noise. The guibo and driveshaft center bearing are affected by age and salted roads too. PS: remove the PCV plenum tube and see if the whistle becomes more obvious. Search forum for how to do that.

https://www.m3post.com/forums/showthread.php?t=1417824
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DrFerry wrote

IF the mechanic isn't taking you seriously go elsewhere. I suggest going to Evolve Automotive who are highly familiar with the E9X M3 platform for diagnosis and analysis. They're in Luton so not too far away from you. The correct OEM E9X M3 diff fluid contains the required FM limited slip additives. Most other generic fluids don't contain FM so it's advisable to check and verify what type of viscosity and brand was used during the service. So, the onset of the noise is possibly not a coincidence. The E9X M3 alternator does make a characteristic whine and clunk when the pulley bearings are going out. The pulley also contains a centrifugal clutch which I have posted about before on these forums. Besides the alternator, the serpentine belt tensioners, as well as the PS pump, and PS pulley as well as the A/C compressor pulley can all make whining or whirring noises. Listen under the hood to better identify where the whistling or whining maybe coming from. Another trick is to use a long screwdriver with the tip placed on each ancillary and your ear touching on the handle end to use it as a sort of stethoscope to locate the noise. The guibo and driveshaft center bearing are affected by age and salted roads too. PS: remove the PCV plenum tube and see if the whistle becomes more obvious. Search forum for how to do that.

Hey, thanks again for your reply.

Agreed, I am looking at going elsewhere. Evolve is a very good shout, there's 1 place closer to me who I will try first, otherwise Evolve is the next plan.

Yep, so I've been using Castrol Syntrax 75W-140 for the last 2 changes now, and haven't had a single issue since it first went in in Feb 2020 (9k miles ago). Used the same one again now. However I am not discounting the potential of this being related to the issue.

Agreed with all of your points, I have even bought my own mechanic's stethoscope to listen out to all of the items in your list, but none of them are giving any obvious clues.

It's very hard to locate the source of the noise when looking at the engine at idle, due to its tone/pitch. Sounds like it's "coming from everywhere". But surely a more experienced mechanic will be able be able to get to it.

My guess of its location from spending a good while listening to it (while battling the fans coming on and hiding the noise every 20 seconds while I try to do this!) is that it sounds like it's coming from somewhere low down in the engine bay, more towards the left (bank 1 side) if I had to guess.

This is matching up with the underside video where it sounds like it's coming mostly from somewhere near the driver-side (UK) DP/manifold area.

At this point, I'm very much with you on suspecting the DP gasket or connection, possibly having a leak of some sort and causing the noise both at idle and when driving. I will suggest this as my personal 'most likely cause' of the issue, but will obviously let the tech do their thing and investigate.

Oh yes I also plan to see what happens if I disconnect that connector at the back of the plenum like you've shown, to see if it helps, BUT the couple of videos I've seen on YouTube of what is likely that plenum whistle issue, sounds really quite different to my issue. Also I can't seem to get my plenum to whistle when I hit it with repeated revs at standstill, it's only at actual idle RPM that you hear it at idle, and with some light load while driving.

Thanks again for your input, I will keep at this and hopefully get it sorted over the next few weeks (weekend car, lack of time, bad weather with salt on roads, all slowing down my schedule for this).

If anyone else has any ideas about what it may be, please do chime in.

Thanks.

Stock intake, filter and plenum?

Mr. C4 wrote

Stock intake, filter plenum?

Yes sir, currently all parts in the engine bay are stock.

The only performance-related mods on the car are:

- Test pipes
- Evolve Stage 2 tune
- OEM backbox mod
- Ohlins Coilovers
- Brembo brake calipers

I am finally booked in with a local garage in 2 days to try and smoke test the exhaust system and the intake.

Will be sure to keep this thread updated as I make any progress.

Update

Did a smoke test of the intake and the exhaust.

No issues found anywhere.

I'm hoping the smoke passed through the secondary cats and into the test pipes area to test the headers>downpipes gaskets, my friend (who was the tech helping me do this) said it usually does.

Also ran the engine with each of the belts off one by one, no change, humming/wooing at idle still there.

We had to end it here as it was after hours and we were already 2 hours in.

Next I'm looking to book in with an M3 specialist to inspect all drivetrain-related bushings and mountings, including the driveshaft CB and guibo etc.

It was confirmed (for the 3rd time, from the 4th tech) that everything is all good and there are no clear problems, the car drives perfectly, it's just a noise (and my OCD wants it gone).